...the innovation we are often missing from Apple, again, comes from a third party developer via Cydia:http://stereopsis.com/flux/
Works on iPhone too, uses practically no battery and: makes a huge difference.
It basically changes the color temperature of the display according to time of day, which makes it emit more natural light at night (esp good for reading in bed, your iPad will not keep you awake)...
Take a couple of days to really get used to it, then you're good. I've first started using this on my Macbook, which has since been replaced by an iPad and I just last week came around to looking for a solution like this and was stunned that it had actually been ported by the developers themselves... Pretty unique app I'd say...
Enjoy!